Variable Primary Flow Chilled Water System Description :The advancement in technology for HVAC controls applied to chilled-water system makes it possible to reduce the environmental impact and operating costs. Chillers are the biggest power consuming equipment in the chiller system but other equipment such as pumps, cooling towers and or dry coolers, cooling tower fans and other motors should also be considered.
A variable flow distribution pumping system to serve the HVAC load is one of the methods to reduce the initial cost and operating cost — depending on the application and the spread area of chilled water load. (speed and pressure modulation is usually accomplished by providing the chilled water pump with a variable frequency drive) In primary variable flow system, chiller water flow rate is varied in the chiller depending on the load. This enables to work on higher deltaT unlike lower delta T syndrome happening in traditional Primary + Variable secondary systems. The effect is reduction in the cost of procuring water and pumping load, translating into savings in initial cost and as well as the operating cost. In primary variable flow systems, evaporator shall be suitable for variable flow operation. i.e. designed to perform proper heat transfer at different water velocities across the tubes. Generally the flow will be varied between 40% to 100% of the design flow, depending on the load condition. Another important factor to be considered while designing primary variable flow system is that the chilled water flow change rate change should not be more than 2% from the previous flow within 10 Minutes of band. Very fast flow variation to the chiller can